spring reverb tanks

Hey,

I've come across a hammond 2 spring reverb tank and reverb transformer which came from a fenton weill (burns?) 18/20watt EL84 amp. I was wondering if by using something like this: http://members.tripod.com/~roymal/ultimate.htm I could make it into a standalone unit...

Cheers,

D

I guess a lot depends on the impedance of the coil on the reverb tank, but I think a JFET input is overkill impedance wise. I would be surprised if the coil is much higher than 10K. A bipolar is probably better suited as far as noise goes, or just use a bipolar input opamp like a 5534. The reverb coil could be used in a balanced input configuration.

After looking at the article further, the author claims a coil impedance of 200 ohms. I disagree that the impedance is the same as the DC resistance, but it will still be quite low, lower than any advantage using a FET would offer.

I dunno if it's anything like the newer accutronics reverb-tanks. But there is a lot of info on the accutronics site www.accutronicsreverb.com . I remember reading the the output impedance rises quite a bit with frequency so it's not a bad idea to use fets here so you won't lose too much here.

There are some schematics on this site too.

I might be wrong, but I doubt that the coil will have any kind of impedance rise high enough to need an input impedance of 100K or higher, and this is where a FET would have positive benefits as far as lower noise. I would still recommend a bipolar transistor input for imedances under 10K.

[quote author="BYacey"]I might be wrong, but I doubt that the coil will have any kind of impedance rise high enough to need an input impedance of 100K or higher, and this is where a FET would have positive benefits as far as lower noise. I would still recommend a bipolar transistor input for imedances under 10K.[/quote]
What the hell. My 2SK170 have some 40 ohms input noise.
Simple measurable, if you solder that resistou between gate and source.
And then measure output noise, and do the some for short.

Modern fets have the some noise performance like bipolar, but
for impedances of several kiloohms you still can use bipolars.
For 10 kohm input, current noise of bipolar is significant.
That blablas about 100 kohms for fets are for fets with 0.1 mA/V transconductancy, some Shockleys experiments and so on. Today we have modern devices
with several cent price...
If you want better noise performance of your FET circuit, use large FET
(2SK170 )and correspondently smaller resistors to set D.C. point.
And you will have problems with noise of 50 ohm input coil

xvlk
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You're probably right. I just looked it up on the accutronics site. They make types with three output coils with impedances of 600, 2k and 10k. I remebered reading a warning about the output impedance rising but that was only for the 10k version

1. Since the output transducers is also essentially inductive, output impedance increases with frequency. Amplifier input impedance should be high enough to prevent premature roll off in the frequency range of interest (i.e., the 10,000 ohm coil appears as 60,000 ohm source at 6kHz).

Click to expand...

Dunno what the original hammond types where. But probably lower

The proof is easy to discover. Build one of each type and see which performs better. I question if modern Fets perform as well as bipolars with low source impedances, I am sure they would be using them for low noise, low impedance mic preamps if they did. After all, the 200 Ohm pickup coil on a spring reverb isn't much different from a 200 ohm dynamic mic coil impedance.